1. Field of the Invention
The present invention relates to a transistor and a method for manufacturing the transistor. In particular, the present invention relates to a transistor in which a channel is formed in an oxide semiconductor layer and a method for manufacturing the transistor. In addition, the present invention relates to a semiconductor device including the transistor.
The invention disclosed in this specification includes in its scope an element formed using a compound semiconductor, in addition to that formed using a silicon semiconductor, as a component of a semiconductor integrated circuit, and discloses an element formed using a wide-gap semiconductor as an example.
In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electrooptic device, a semiconductor circuit, and electronic equipment are all semiconductor devices.
2. Description of the Related Art
Electronic devices and optical devices are manufactured using transistors including silicon. Other than a transistor formed using silicon, in recent years, a technique in which a transistor is formed using an oxide semiconductor and applied to an electronic device or an optical device has attracted attention.
In an active matrix liquid crystal display device, a transistor is provided in each pixel. Such transistors are mostly formed using amorphous silicon, polycrystalline silicon, or the like. Note that a transistor formed using amorphous silicon can be applied to a large-sized liquid crystal display device although the transistor has low field-effect mobility. In contrast, it is difficult to apply a transistor formed using polycrystalline silicon to a large-sized liquid crystal display device although the transistor has high field-effect mobility.
As an alternative material of a transistor to a silicon-based material, an oxide semiconductor has attracted attention. This is because a transistor which is formed using an oxide semiconductor has higher field-effect mobility than a transistor which is formed using amorphous silicon and is applicable to a large-sized liquid crystal display device. For example, Patent Document 1 and Patent Document 2 disclose a technique by which a transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide, which is an oxide semiconductor, and is used as a switching element of a pixel or the like of a display device.
Patent Document 3 discloses a transistor including a stack of oxide semiconductor films in which an amorphous component remains at an interface with a base.
Note that it is difficult to control the threshold voltage of the transistor formed using an oxide semiconductor. Specifically, in an oxide semiconductor, part of hydrogen serves as a donor to release electrons as carriers. Then, the carrier density of the oxide semiconductor is increased, so that a channel is formed in the oxide semiconductor even without application of voltage between the gate and the source of the transistor. In other words, the threshold voltage of the transistor shifts in the negative direction.
When the threshold voltage of the transistor is high or negative even when the field effect mobility is high, it is difficult to control a circuit including the transistor. In the case where a transistor has a large threshold voltage value and a large absolute value of the threshold voltage, the transistor cannot perform a switching function as a transistor and might be a load when a transistor is driven at low voltage. Further, when the threshold voltage value is on the minus side, current tends to flow between the source and drain electrodes even if the gate voltage is 0 V, that is, the transistor tends to be normally on.
In the case of an n-channel transistor, it is desirable that only after a positive voltage is applied to a gate electrode, a channel be formed and a drain current flows. A transistor in which a channel is not formed unless the driving voltage is increased and a transistor in which a channel is formed and drain current flows even in the case of the negative voltage state are unsuitable for a transistor used for a circuit.
Note that it is difficult to completely remove hydrogen contained in the oxide semiconductor. Therefore, the control of the threshold voltage of the transistor formed using an oxide semiconductor is more difficult than that of the threshold voltage of the transistor formed using a silicon-based material.